1. Field of the Invention
This invention relates to a gas detection tape optimum for optically determining the concentration of a hydride gas from the trace of reaction between the hydride gas and a coloring reagent on a cellulose tape.
2. Description of the Prior Art
Being colorless , metallic hydrides, such as phosphine, silane, arsine and diborane, are very difficult to detect by the five senses of human beings. In the semiconductor industry where these highly toxic hydride gases are used, as such, provision of gas detectors to indicate their existence is compulsory. Common electrochemical or semiconductor gas sensors for other ordinary gases are unsuited because their sensitivity to hydride gases are extremely low. Usually, tapes that color when they come into contact and chemical reaction with hydride gases are used. The concentration of hydride gases is determined by measuring the optical density of the trace of their reaction.
Silver nitrate is a well-known reagent that colors when reacting with hydride gases. A hydride gas detection tape with silver nitrate comprises silver nitrate placed on a gas-permeable cellulose tape so that silver nitrate leaves a trace of the reaction of a colloid of silver on being exposed to and reduced by a hydride gas. The concentration of a hydride gas can be easily determined with high accuracy by measuring a change in the reflectance of the light thrown upon the trace of reaction formed on the tape.
However, silver nitrate is so sensitive to light that it is used as the coating emulsion on photographic films. As such, hydride gas detection tapes with silver nitrate must usually be kept in dark containers. Still, tapes once exposed to light become brown in about 24 hours even if they are kept in dark containers.
To eliminate this shortcoming, a detection tape with an improved light resistance was proposed (Japanese Provisional Patent Publication No. 99753 of 1983). The improved proved light resistance is obtained by reducing to a minimum the light-induced production of a colloid of silver by adding such strong acid as nitric acid to a cellulose tape impregnated with silver nitrate.
This type of hydride gas detection tapes prepared by the addition of silver nitrate remain intact enough to produce no error in measurement for about half a year when kept in dark containers.
However, nitric acid or other strong acid added for the improvement of light resistance attacks the cellulose holding the coloring reagent, thereby seriously impairing the mechanical strength of the detection tape over a long period of time.
The reduction of cellulose strength present a serious problem to the use in automatic measuring devices having a built-in gas sampler or optical gas concentration detec-tor. In this type of measuring device, the unused part of a hydride gas detection tape is exposed to the measuring area at given intervals to perform automatic measurement. For this reason, the detection tape set in the measuring device is passed over a storage reel and a take-up reel so that the tape can be taken up through the measuring area when required. At each measuring time, a given length of the unused part of the tape is paid off into the measuring area and, then, brought into contact with a sampled gas. Paid off under a considerable tension, the detection tape might break if it does not have adequate mechanical strength.
To provide a solution to this problem, the inventor proposed a hydride gas detection tape that has an improved light resistance without impairing the mechanical strength of the reagent holder. This hydride gas detection tape is prepared by using para-toluenesulfonic acid instead of nitric acid or other strong acids (Japanese Provisional Patent Publication No. 275352 of 1990). As no discoloration or nitration of cellulose occurs, the tape invariably remains the same as it was manufactured and thus permits a highly reliable automatic measurement of hydride gas.